A porous material that can be produced with a biological material, such as a protein and a saccharide, is utilized in various fields in industries, for example, a medical field, such as a wound coverage, a hemostatic sponge and a pharmaceutical sustained-release carrier, daily goods, such as a disposable diaper and a sanitary napkin, a field of water purification where the porous material may be used as a support as a habitat of microorganisms and bacteria, a cosmetic and beauty treatment field aiming at moisture retention and the like by using the porous material personally or in an esthetic salon, and cell culture substrate and a tissue regeneration substrate in a tissue engineering and regeneration medical engineering.
As the biological material constituting the porous material, saccharides, such as cellulose and chitin, and proteins, such as collagen, keratin and silk fibroin, are known.
Among these, collagen has been most frequently used as a protein, but it is becoming difficult to use bovine collagen since the generation of BSE disaster. Keratin, which may be obtained from wool or feather, has a problem in availability of the raw materials thereof, and is difficult to use industrially. The raw materials of keratin is difficult to procure since wool is increased in raw material price, and feather has no market. Silk fibroin, on the other hand, may be utilized industrially since it is expected to be available stably from the standpoint of availability of raw materials, and is stable in price.
Silk fibroin has been used as surgical suture threads for a prolonged period of time in addition to the clothing purposes, and is also used currently as an additive for foods and cosmetics, owing to the safety for human body, and thus silk fibroin may be sufficiently utilized in the field of the porous material.
There has been several reports on a method for producing a porous silk fibroin material. In one method, for example, a silk fibroin aqueous solution is quickly frozen and then immersed in a crystallization solvent, thereby performing melting and crystallization simultaneously (Patent Document 1). In this method, however, an organic solvent as the crystallization solvent is necessarily used in a large amount, and the possible contamination with residual solvent may not be negated, which result in problems on the use in the application fields including the aforementioned medical field and the like. In another method, an aqueous solution of silk fibroin is gelled by maintaining the pH thereof at 6 or less or gelled by adding a poor solvent to the aqueous solution, and the resulting gel is freeze-dried, thereby providing a porous material (Patent Document 2). However, the method may not produce a porous material having sufficient strength. Furthermore, a method has been reported that a silk fibroin aqueous solution is frozen and then maintained in the frozen state for a long period of time, thereby providing a porous material (Patent Document 3). However, the investigations made by the present inventors reveal that the method is poor in reproducibility, and a porous material may frequently not be obtained.
A method that is reliable and convenient as compared to the aforementioned methods for producing a porous silk fibroin material has been reported (Patent Document 4 and Non-patent Document 1). In this method, a small amount of an organic solvent is added to a silk fibroin aqueous solution, which is then frozen for a prescribed period of time and then melted, thereby providing a porous silk fibroin material. In this method, the organic solvent used in a small amount is removed by a rinsing step using ultrapure water providing substantially no residual solvent, and the resulting porous material in a moistened state has higher strength and is excellent in shape stability as compared to the ordinary porous materials reported.